The present invention relates to a fuel assembly for a boiling water reactor comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate and being surrounded by a fuel channel with a substantially square cross section.
In a boiling water nuclear reactor, moderated by light water, the fuel is in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice, and is retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods spaced from each other and prevent them from bending or vibrating when the reactor is in operation, a number of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each of which extends along the main part of the length of the fuel assembly, surrounded by a substantially square fuel channel.
A core in a boiling water reactor comprises several hundred fuel assemblies arranged vertically in the core in a certain spaced relationship to each other. The fuel assemblies are arranged in a symmetrical lattice with each fuel assembly included in two rows of fuel assemblies perpendicular to each other. The core also comprises control rods with four blades, extending perpendicularly to each other from a central part and forming a right-angled cross. The control rods are arranged with each of their blades between two fuel assemblies located in the same row, such that each control rod together with four fuel assemblies arranged around its fuel blades forms one unit.
The core is immersed into water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby part of the water is transformed into steam. The percentage of steam increases towards the top of the fuel assembly. In the lower part of the fuel assembly, the coolant is in singlephase state and in the upper part thereof in two-phase state. This difference between the upper and lower parts gives rise to special problems which must be taken into account when the fuel is formed, such as the following:
the pressure drop is several times higher in the upper part of the fuel assembly than in the lower part thereof, which, among other things increases the risk of thermohydraulic instability;
there is a risk of dryout in the upper part of the fuel assembly but not in the lower part thereof; and
the neutron moderation is less effective in the upper part, which leads to the fuel not being burnt up as quickly in the upper part of the fuel assembly as in the lower part thereof.
Therefore, it is desirable to achieve a fuel assembly which in a simple manner can be designed where the upper part of the fuel assembly differs from the lower part, and where the fuel distribution and the free flow area may be varied in the axial direction to obtain optimum conditions.
One problem with conventional fuel assemblies for boiling water reactors is that they are not sufficiently flexible and that it is therefore difficult to give them different designs in the upper and the lower part. It is also difficult to obtain an optimum fuel distribution in the axial direction.
A fuel bundle for a boiling water reactor is normally about four meters in length and has a width which varies between 0.05 and 0.2 mm. The considerable length of the fuel bundle in relation to the width thereof entails difficulties in the manufacture, transport, handling, and storage of the fuel bundles. Another disadvantage with full-length fuel rods is that they contain large quantities of uranium and harmful fission products. This means that fuel damage to a fuel rod may have serious consequences since considerable quantities of uranium and fission products risk leaking out into the coolant.
Experience shows that, for example in connection with repairs and service of a nuclear reactor, debris may enter, for example metal chips, which then move with the water which circulates through the core. It has been found that these debris may give rise to abrasion damage to the cladding tube. The abrasion damage normally arises on a level with the spacers because the debris adhere to the spacers and remain there and subject the cladding tube to wear. Penetrating water then tends to give rise to considerable secondary damage, often far away from the primary site of the damage.
British patent specification No. 1 403 491 shows a fuel element for a nuclear reactor containing a plurality of short fuel units, each one consisting of a plurality of fuel rods arranged in parallel with each other between a top tie plate and a bottom tie plate. The fuel rods are also supported by intermediate spacers. The fuel units are fitted onto a supporting tube in such a way that the bottom plate of one fuel unit rests on the top plate of the other, and that the fuel rods in each fuel unit are parallel to the fuel rods in the other fuel units. The supporting tube with the fuel units extends through the whole fuel assembly. The fuel units have a substantially circular cross section along their longitudinal axis. This fuel element is intended to be used in a heavy-water moderated nuclear reactor. A heavy-water moderated nuclear reactor comprises a plurality of pressure-supporting cooling channels. During their operating period, the fuel elements are inserted into these cooling channels.
Another embodiment of short fuel units is known from Canadian patent specification No. 1 250 966. Each fuel unit is sufficiently short to eliminate the need for intermediate spacers. Also these fuel units are primarily intended to be used in a heavy-water moderated reactor, especially with pressure tubes, and have a substantially circular cross section along the whole of their length.
The need of a flexible fuel assembly for a boiling water reactor is connected with the problems which arise due to the waterxe2x80x94which is both the moderator and the coolantxe2x80x94boiling and the special requirements therefore made on the upper and lower parts of the fuel assembly. These problems have been known for a long time (since the early 60""s).
Using short fuel units in a heavy-water moderated reactor has been known since the childhood of nuclear engineering. The conditions for a heavy-water moderated and a light-water moderated nuclear reactor are, however, very different; for example, a heavy water moderated reactor has a low burnup and a high linear power density compared with a light-water moderated reactor which has a high burnup but a lower linear power density. A decisive difference in comparison with a light-water moderated reactor of boiling-water type is that the moderation is completely dominated by a large volume of separate moderator water, which is not allowed to boil in current heavy-water reactors. This means that the moderation is essentially constant in the axial direction along the fuel units.
Nor is the cooling water allowed to boil in current heavy-water reactors. For that reason, there is not the same need of an axial variable fuel design.
The invention provides a fuel assembly for a boiling water reactor, moderated by light water, wherein:
it is simple to give the fuel assembly different designs in the upper and the lower part;
flexibility is achieved for optimization of fuel distribution in the axial direction, both initially and in partially burnt-up state;
flexibility is achieved for lattice optimization both in the axial direction and in the radial direction;
the consequences in case of cladding damage, in the form of leakage of fuel and fission products, are smaller than for a traditional fuel;
the risk of abrasion damage caused by debris in the cooling water is smaller than for a traditional fuel; and
the manufacture, transport, handling, and storage of the fuel assembly are simplified.
According to the invention, this is achieved by dividing the fuel bundles into a plurality of short fuel units. Compared with heavy-water reactors and the applications known there-from, light-water boiling reactors place essentially different requirements on the design of the fuel:
the fuel units shall constitute a constructive part of a retained fuel assembly with associated fuel channels with a substantially square cross section;
a burnup several times higher must be attained without the fission gas pressure in the short fuel rods becoming too high;
the thermohydraulic requirements during boiling must be satisfied by actions to promote the utilization of the coolant in the upper part of the fuel assembly, and by limiting the pressure drop there; and
the nuclear requirements during boiling in combination with light-water moderation must be satisfied by ensuring moderation internally in the assembly, and by adapting the distribution of fissile material axially in the assembly.
What characterizes a fuel assembly according to the invention will become clear from the appended claims.